Why is the knowledge of growth and development necessary in the educative process?
Without a working knowledge of human growth and development, a teacher cannot know if a curriculum or objective is developmentally appropriate. While it is true that most educators are guided by local, state or national curriculum goals and objectives already developmentally aligned to grade levels, not all educators have access to such curriculum. Educators left to create their own curriculum must understand the developmental building blocks of learning that create, for example, number sense in math, without which students cannot be successful in higher level math. Even when provided a developmentally correct curriculum, educators must understand the stages of growth and development and be prepared assess where along a curriculum continuum a student is at so that a starting point for teaching can be determined. It is possible that a student has succeeded in attaining promotion to a grade level without having mastered the prerequisite skills he should have in order to be successful. If this is the case, the teacher must be prepared to determine a starting point for reteaching a student those prerequisite skills and locate resources with which to teach them.
The human brain does not emerge from the womb fully formed. Indeed, humans develop more slowly than almost any other animal on Earth. While deer for example can walk a few minutes after being born, humans often take more than a year to do so.
The very slow pace of human brain development has the advantage of expanding our ability to learn, but it also has the disadvantage of making it difficult for us to process certain types of information before we reach the appropriate age.
I think the most important application of this knowledge is in curriculum design for students of various ages.
For example, a proper understanding of human brain development would lead us to put foreign language education very early in the curriculum--preferably in elementary school--because language is one of the first systems to come online and the most fluent speakers of any language are always those who started learning it from a very young age.
Conversely, it would tell us to put off most forms of mathematics education until at least middle school, because the brain is not yet ready to deal with the high levels of abstraction required for algebra and advanced geometry. Of course we can still teach arithmetic and basic geometry, but subjecting fourth graders to algebra is unlikely to help anyone.